The present invention is concerned with head positioning in a disk apparatus, such as a magnetic disk drive apparatus and an optical disk drive apparatus. Especially, the present invention relates to a disk apparatus that moves a head with high speed by position control and to a method of controlling the position of a head in a disk apparatus.
The method of positioning a head on the basis of servo information in a conventional disk apparatus typically involves a seek operation and a follow operation. A seek operation is an operation in which a head is moved at high speed on a virtual track cylinder to a target position. A follow operation is an operation in which the track center of a target is followed with high precision.
The seek time (time required to move a head to a target track during a seek operation) and the positioning precision (precision of a head during a follow operation) are the important elements that determine the storage capability and apparatus performance in a magnetic disk apparatus.
A technique that does not need a change in control mode to speed up an operation in which a short span is sought and that does a seek operation only by position control is disclosed in Japanese patent Laid-open Publication 9-82050 official gazette and Japanese patent Laid-open Publication 9-139032 official gazette.
Many head control systems are known to operate in the following way. A position signal is detected from a signal read by a head. The manipulated variable of a head is calculated by a microprocessor on the basis of the detected position signal. The microprocessor system then outputs the manipulated variable via a power amplifier to move the head. That is, the microprocessor system outputs one manipulated variable for each sampling period.
It is important to shorten the sampling period to achieve high speed movement of the head and high precision positioning of the head. For a disk drive, for example, a magnetic disk drive, storing a lot of positioning data on the recording area on the disk undesirably affects the storage capacity for recording information data, because the larger the positioning data area is, the smaller the information data storing area becomes.
Japanese patent Laid-open Publication 9-82050 discloses a system which outputs many more control signals from a controller per unit time than the number of times which a head crosses a position data area on the disk. In this system, the disk drive has a disk control controller that includes a feedforward controller and a feedback controller to position a head. At the end of a calculation by the feedback controller or the feedforward controller, the disk control controller outputs a periodic control signal.
The above-mentioned controller systems are insufficient in the following points.
(1) To enable the head to be moved with a high speed, the above-mentioned control systems carry out a seek operation using a target acceleration trajectory that is a target trajectory relative to the head acceleration and a target position trajectory that is a target trajectory relative to the head position. But, in these control systems, no consideration is given to discreteness or a delay in the control at the time of designing each target trajectory. In addition, at the time of designing a target trajectory, the calculation time of the feedforward controller and the calculation time of the feedback controller have not been taken into account.
Therefore, in the above-mentioned control systems, it is necessary to correct the target position trajectory one after another after setting up a target position trajectory correction value separately determined through trial and error. And, it is necessary in these known control systems, for example, to output control values to a D/A converter sequentially at the end of each calculation by the controller.
(2) The above-mentioned control systems performs a multirate control that effects a processing with a predetermined timing by sampling 1/2 of the sampling that acquires a position signal as the timing in which a target acceleration trajectory is output. These control systems determine the timing of a multirate output signal by software processing. Therefore, there is a problem in the following points.
It is impossible to set an accurate multirate sampling period.
It is difficult to optionally set the number of multirate outputs that are output in one sampling.
(3) In the above-mentioned control systems, it is difficult to further shorten the seek time, because the target acceleration trajectory has the form of a symmetrical triangular waveform during acceleration and deceleration. Therefore, in these control systems, it is necessary to set the movement period of a target trajectory at more than double the resonant period of the head support mechanism.
The above items represent important problems that should be solved in the design of a high-speed head positioning system in a disk apparatus.